(1) Field of the Invention
The present invention relates to a switched mode power supply (hereafter referred to as SMPS), and in particular to an SMPS having a discharge circuit for causing the output voltage of an auxiliary power source (for supplying an operating power to a switching signal generator and a protective circuit) to drop quickly on turning off the power source.
(2) Description of the Prior Art
An SMPS includes a power supplying device for rectifying the commercially supplied alternating voltage. Current in the primary winding of a switching transformer is switched on and off according to the rectified output voltage of the secondary winding of the transformer. Since such an SMPS has a low ripple content in its output, it is widely used in computer systems or a machinery where a precise power supply is necessary.
Such an SMPS has generally adopted a voltage mode type of a kind of pulse width modulation method, but recently is changing to adopt a current mode type.
The prior art SMPS of current mode type has a disadvantage that, in the case where a person turns the power source off and then on momentarily or operates in response to a disorder of various systems which are connected to the SMPS, and thus the power is turned off, it takes considerable time to re-operate the system. The prior art SMPS of a current mode type will be described below in detail, referring to FIG. 1.
As illustrated, the SMPS of a current mode type includes a primary rectifying circuit 1 having a bridge-type rectifier BD1 for full-wave rectification of alternating voltage applied through a plug PL1 and a capacitor C1 connected between the output of the rectifier BD1 and ground.
The output terminal of the primary rectifying circuit 1 is connected to a primary winding T11 of a switching transformer T1 of which a secondary winding T12 (on the load side) is connected to a secondary rectifying circuit 2 for supplying operating power to the load side and including a forward diode D1 and a capacitor C2 connected between the anode of the diode D1 and ground.
In addition, the transformer T1 further comprises an auxiliary secondary winding T13 which is connected to an auxiliary power source circuit 3 for supplying an auxiliary power source and including a forward diode D2 and a capacitor C3 connected between the anode of the diode D2 and ground.
The output voltage from this auxiliary power source circuit 3 is supplied as an operating power source to a switching signal generator 4 and a protective circuit 5. And the output signal from the switching signal generator 4 is supplied to the gate of a field effect transistor FET1 connected between the primary winding T11 of the transformer T1 and ground, so that the field effect transistor FET1 turns on and off alternately.
A starting resistor Rs is inserted between the output of the primary rectifying circuit 1 and the power source terminal of the switching signal generator 4. This starting resistor Rs forms a supplying path of an initial power source for driving the switching signal generator 4 on the initial operation.
A voltage divider 8 comprising resistors R1, R2 connected in series is inserted between the output terminal of the secondary rectifying circuit 2 and ground. The voltage applied between the resistor R2 of the voltage divider 8 and ground is supplied to a voltage fluctuation sensing section 6, and this voltage fluctuation sensing section 6 drives the light emitting portion of an optocoupler PC1 connected to the output side, according to the input signal from the voltage divider 8.
Furthermore, the output of the light receiving portion of the optocoupler PC1 is connected to the switching signal generator 4 so that the switching signal generator 4 can control the field effect transistor FET1 according to the operating state of the optocoupler PC1, and is also connected to the protective circuit.
The operation of the SMPS having the above-mentioned construction is explained below in detail.
The alternating voltage applied to the plug PL1 is rectified in the primary rectifying circuit 1 to output the direct current DC voltage.
DC current output from the primary rectifying circuit 1 supplies the switching signal generator with an initial power source through the starting resistor Rs to drive the field effect transistor FET1 and flows through the primary winding T11 of the transformer T1 to induce a voltage to the secondary winding T12 of the load side and to the secondary winding T13 of the auxiliary side. AC voltage induced in the secondary winding T13 of the auxiliary side is rectified to DC voltage through the auxiliary power circuit 3, which is applied to the power source terminal of the switching signal generator 4 and the protective circuit 5, and accordingly the switching signal generator 4 normally works to output pulse signal to the field effect transistor FET1.
Therefore, the field effect transistor FET1 turns on and off alternately according to the pulse signals which are output from the switching signal generator, and these reiterations of on-off operation of the field effect transistor FET1 make the output voltage of the primary rectifying circuit 1 applied to the primary winding T11 of the transformer reiterate on and off.
The voltage is intermittently induced in the secondary winding T12 of the load side and to the secondary winding T13 of the auxiliary side by said reiterations, and the voltage induced in the secondary winding T12 of the load side is rectified through the secondary rectifying circuit 2, thereby supplied to the operating power source of the load side.
In such a state, since the voltage applied between the resistances R12 of the voltage divider 8 and ground is input to the voltage fluctuation sensing section 6, the voltage fluctuation sensing section 6 changes the brightness of the light emitting portion of the optocoupler PC1 in accordance with the level variation of the operating power supplied from the secondary rectifying circuit 2 to the load side. The light receiving portion detects the brightness of the light emitting portion, which is received by the switching signal generator 4 and the protective circuit 5.
In consequence, the switching signal generator 4 controls the operating cycle of the field effect transistor FET1 according to the level variation of the operating power supplied to the load side, thereby preserving the operating power supplied to the load side at a certain level. If the level-variation of the operating power is too large to be controlled, the protective circuit 5 works to stop the operation of the switching signal generator 4 so that the operating power for the load may not be at an excessive level, thereby protecting the machine connected to the load side from damage.
In such an SMPS of a current mode type, when the power turns on again after a momentary turn-off resulting from a sudden power stoppage or when the user cuts off the power as the operation of the protective circuit 5 on account of an abnormal operation of the load side causes a stop of the switching operation of the switching signal generator 4, a residual electric charge will remain in the capacitor C3 of the auxiliary power circuit 3.
In consequence, the SMPS does not work until the residual electric charge is discharged to a voltage less than a certain level, since the protective circuit 5 continues to operate even after turning on again the power switch. Accordingly, there exists a disadvantage that it takes about 5 to 7 seconds to discharge the residual electric charge of the capacitor C3 and to reoperate the SMPS.